Microscopes employing conventional optical imaging systems are limited in their resolution capabilities. It is known that conventional optical microscopy techniques can not be used to resolve features significantly smaller than one-half the wavelength of the light used to illuminate the sample. As a result, transmission and scanning electron microscopes were developed in order to provide the ability to examine structures substantially smaller than the wavelength of visible light. In fact, technologies such as scanning tunneling microscopy (STM) have allowed the resolution of structures as small as individual atoms.
Unfortunately, these high resolution techniques suffer from the drawback that they require that the sample be placed in a vacuum and/or be subjected to ionizing radiation. This requirement has proved unsatisfactory for many types of specimens (e.g. biological materials) since considerable damage to the specimen or modification of the property to be investigated often results from observation or sample preparation. Moreover, most of these techniques employ tunnelling electrons or an electron beam as the signal source within the microscope. Thus, a sample must be generally electrically conductive in order to be observable.
In addition to these problems, microscopes based upon tunnelling electrons are unable to fulfill the requirements of researchers who wish to study the electrical or optical properties of an object. Certain features of a specimen that are detectable by optical microscopy may nevertheless be invisible to an electron microscope because the two devices measure substantially different physical properties. For example, while electron microscopes are suited quite well for examining surface topology, they are practically unusable for studying electro-optic and/or optical properties of specimens such as active semiconductor devices or biological samples. SEMs (scanning electron microscopes), TEMs (transmission electron microscopes) and STMs provide primarily structural information. These types of instruments can not adequately provide information on a specimen's optical properties such as color, reflectance, fluorescence and luminescence.
In response to these problems, near-field scanning optical microscopy (NSOM) has been developed to achieve fine resolution (well below the one-half wavelength diffraction limitation) without any resultant damage to the observed sample. NSOM is a relatively recent technology that has been employed for both the imaging and spectroscopy of materials at resolutions far below the familiar diffraction limit. NSOMs are capable of measuring a variety of optical properties associated with a sample. An NSOM generally consists of an aperture having a diameter that is smaller than an optical wavelength which is positioned in close proximity to the surface of a specimen and scanned over the desired portion of the sample. The light thus exiting the aperture is largely independent of the wavelength of the incident light.
As the aperture is moved across the sample, an optical response of the specimen to the near-field is produced, and the resulting photons are detected by a remote photodetector. Conventional means are then employed to collect and assemble data such that a scanned image corresponding to the sample is produced for viewing.
NSOMs generally require some method for determining and maintaining a particular distance between the probe tip and the sample surface. This is often referred to as z-axis control. Shear force topographic imaging (dithering) has emerged as one technique for use in NSOMs. This method is sensitive, non-destructive, sample independent and provides a wide dynamic signal range for distances up to 50 nm above the sample. Typically, the tip is dithered by mounting it on a piezoelectric tube. As a result, prior art devices employing the dithering technique for z-axis control have heretofore used off-axis objectives for the collection and illumination of the tip region.
The imaging capabilities of super-resolution devices such as the NSOM are desirable in a broad range of disciplines ranging from semiconductor devices and materials to biological systems and beyond. For example, coupled with sensitive spectroscopic probes, NSOM can provide an unprecedented level of diagnostic capabilities to investigate and understand the optical and electro-optic properties of active semiconductor devices on a better than 30 nm length scale (.lambda./20) in the visible light region. Additionally, optical modes in optoelectronic devices can be mapped, local doping profiles can be determined and fabrication process and lattice mismatch induced strains can be ascertained.
NSOM devices further provide the ability to map photoluminescence (PL) and electro-luminescence (EL) emission at subwavelength resolution. PL can determine defect type and density relative to band edge emission by examining intensity ratios. PL wavelength shifts in band edge emission are indicative of local strain fields. EL is used to understand the behavior of active opto-electronic devices. Additionally, using NSOM in illumination-transmission mode (discussed below), single molecules can be imaged using near-field fluorescence microscopy. Site specific near-field fluorescence microscopy can provide novel information on biological systems.
Often, the aperture in NSOM devices is provided in the form of a tapered single mode optical fiber with a typical aperture of 20-200 nm. The fiber tip is placed within the near optical field of the sample. Because both the tip to sample separation and the tip aperture are a small fraction of the visible light wavelength, the resulting spatial resolution is not limited by the usual far field Rayleigh criteria of .lambda./2. In NSOM, the electric and magnetic fields at the sample are effectively confined to the tip diameter, and therefore can yield resolutions as high as .lambda./40, or about 15 nm for visible wavelengths.
NSOM devices operate primarily in one of two distinct modes. In the first possible mode, illumination mode, the excitation light is directed down the tapered optical fiber tip, and the transmitted, reflected or emitted light is collected by far-field optics. In the second mode, collection mode, the sample is excited by far-field optics, and the transmitted, reflected or emitted signal is collected in the near-field by the fiber tip. Collection mode operation is typically employed when examining semiconductor and opto-electronic systems. This is because when examining these types of samples, excitons diffuse from the excitation point prior to recombination. It is thus beneficial to use the near field resolution to collect light rather than to excite the sample. In contrast, biological systems are better suited to illumination mode, since the fluorescence emanates from localized centers.
NSOM operation can be further characterized according to directional relationship by which light is collected. In a first procedure, incident light (produced either in illumination mode or collection mode) is transmitted through the sample and collected below the stage. The collected light is directed towards a photodetector device and the image is reconstructed. This method is referred to as transmission mode and is commonly employed with transparent or semi-opaque samples such as biological specimens. Alternatively, an NSOM may operate according to a reflection mode whereby light is reflected or emitted from the sample surface and collected either in the near field (collection mode) or in the far field (illumination mode) somewhere above the sample surface.
Opaque samples require the use of reflection mode. One reflection mode technique which has been used with some degree of success has been suggested by R. D. Grober et al. ("Design and Implementation of a Low Temperature Near-Field Scanning Optical Microscope", Rev. Sci. Instr., March 1994). Grober calls for placing an optical fiber tip at the focal point of a reflecting objective. The resulting apparatus provides dispersionless optics using a microscope objective having a small primary convex mirror and a large secondary concave mirror. The mirrors are mounted on plates capable of vertical motion for focusing and collimating the luminescence. Grober has reported that he has been able to achieve a numerical aperture (NA) value of 0.4.
The Grober device, however, suffers from a number of disadvantages. Firstly, the mirroring apparatus requires a motion control separate from the typical x, y and z dimension motion controls used to move the sample platform and/or the fiber probe. As a result, the device is more costly than a non-reflecting objective based counterpart device. Moreover, the inclusion of the mirrors and their associated motion controls increases the physical dimensions of the optical system contained within the NSOM device. An assembly that is not compact in size is often impossible to incorporate within an existing conventional microscope. In addition, specialized environments such as vacuums and cryogenic chambers often can not accommodate a bulky assembly such as that required with the Grober design.
As described above, opaque specimens require the use of a reflection mode NSOM device. Heretofore, the operation of NSOMs in reflection mode has occurred almost exclusively through the use of off-axis collection objectives. One known exception is a device described in U.S. Pat. No. 4,725,727 issued to Harder et al. This patent appears to describe a generally co-axial scheme using a waveguide formed from quartz crystal. A tip is formed and two opaque layers are deposited on the tip such that, for example, an inner transparent layer may be used to illuminate the sample, with the reflected light being captured by the outer layer and delivered to photodetectors. This scheme, however, requires multiple, complicated coatings on the tip and is thus difficult to implement in practice. In addition, this scheme can not perform any imaging of the sample region about the tip to, for example, direct the tip over the sample region to be studied.
FIG. 1 illustrates a prior art near-field optical microscope operating in the reflection mode. An example of an NSOM using such an off-axis objective in a reflective geometry is the Aurora TMX2000 model built by the Topometrix Corporation located in Santa Clara, Calif. For purposes of illustration, however, a distinct prior art reflective mode NSOM is shown in FIG. 1 and is discussed herein.
As is apparent to one of ordinary skill in the art, the prior art NSOM of FIG. 1 includes a probe 10 terminating in a probe tip 70. A stage 20 is further provided for supporting sample 30. The probe may be displaced relative to the sample in the x, y and z dimensions by means of piezoelectric actuators 40. A light source 60 is employed to illuminate probe tip 70 and a photodetector 80 is provided for the detection of a change in amplitude or phase of the vibrating (dithering) tip.
Off-axis, side mounted objective lens 120 is used to image the tip region, provide illumination, or collect the reflected or emitted light from the sample as a result of illumination through probe 10 and tip 70 by way of source 60. In a typical off-axis scheme, the objective lens 120 may be mounted at a distance of approximately 10 mm from the tip 70.
Such an off-axis scheme suffers from a small collection efficiency and reduced resolution. This is because the use of an off-axis objective requires independent x, y, z controls at the off-axis objective. Because of this design, a relatively large distance between the sample 30 and objective 120 is needed. This, in turn, results in a smaller numerical aperture for collecting the reflected or emitted light resulting in decreased performance. Further, as a result of this reduced collection efficiency, scan speed is generally reduced in order to achieve a satisfactory resolution. Alternatively, if scan speed is maintained, the resolution will suffer.
In contrast, if it is possible to place the collecting device (i.e. the collection objective) in co-axial alignment with the probe tip, the collection efficiency and thus the resolution and brightness will improve dramatically. This is because a higher numerical aperture can be achieved by co-axial detection as a result of a reduction in distance between the probe tip and the collection objective. For example, this distance may be reduced by a factor of approximately five to 2 mm by using a co-axial design.